How are Microgrid Networks Optimized to Preven Voltage and Frequency Droop
Short Answer With the advent of multiple small generators and energy storage devices in the modern power network, innovative solutions to the unique problems and opportunities of these new systems must be found. One of these problems (or opportunities) is the issue of how to correct voltage and frequency droop in distributed Microgrids. This paper describes some of the methods that can be used to help mitigate this issue. Smartgrids are collections of various power producing, storage, and distribution systems that are controlled in a manner to optimize the system in a number of different ways. Micorgrids are smaller potentially self-contained systems that may consist of Distributed Generators (DGs), storage units, and local distribution lines. Microgrids interface with the Maingrid through a Point of Common Connection (PCC). The DGs within the Microgrid may consist of Photovoltaic (PV) cells, wind generators, small fuel burning units, or other power producing units. The energy storage units may consist of Uninterruptable Power Supplies (UPSs), capacitor banks, batteries, or electrical cars connected to the power network. Within Microgrids, the distribution system is usually transmitted at a relatively low voltage in comparison to the voltages that can be transmitted throughout the Maingrid. In the case that there is a fault or generation failure in the Maingrid, Microgrids have the capability to isolate themselves from the Maingrid and perhaps maintain power within them by a process referred to as Islanding. Loads within the system can come online or go offline and when this occurs, it will cause fluctuations within the power system. This is seen as changes in the voltage or frequency levels. These changes are referred to as droop. In order to counteract the droop introduced by the changing power requirements, a control system that has the ability to adjust the power provided by DGs and storage units may be implemented. The power in a power system consists of two types of power, Real power (P) that is consumed by resistive loads, and Reactive power (Q) which is consumed by inductive or capacitive loads. The sum of these two powers is called the Apparent Power (S). (1) If power is transmitted across a lossless line (one without a resistive load) the real and reactive power can be related to the voltages on either side of the line and to the power angle by (2) and (3). (2) (3) Since the power angle will be small, we can say that and . This allows (2) and (3) to be rewritten as (4) and (5). (4) (5) A number of Distributed Generators (DGs), which may contain various power producing units such as Photo Voltaics (PVs), batteries, wind generators, fuel cells, or fossil fuel burning generators are connected within microgrids. These in turn are connected to the main power grid. Two main objectives for control and optimization of the power system are required. First, the control of the various DGs is necessary to provide stability by keeping the voltage and frequency profile within an acceptable range. Second, the optimization of the cost/profit to the main grid and the microgrid operators will be determined. To insure that the voltage/frequency droop control is kept within the nominal range, the DGs will work cooperatively together utilizing intermittent, asynchronous, low bandwidth links. By sensing the local power conditions and by communication with other DGs, the local DG can determine its individual output requirements. To determine the cost/profit between the microgrid operators and the main grid power provider, an optimization problem is presented. Both the microgrid and the main grid operators will want to maximizes their profit (minimize their cost), but this must be accomplished within the requirement of grid stability. In other words the DGs must supply power at peak times and store power at lulls. To model this optimization, different game theory methods with be used to find optimal bidding strategies. Specifically, the Nash game for interaction between the DGs and the Stackelberg game for the interaction between the microgrids and the main grid will be used.